Embodiments of the inventive concepts described herein relate to radiation-tolerant unit metal-oxide field-effect transistors (MOSFETs) and more particularly, relate to unit MOSFETs having radiation-tolerant characteristics hardened against single event effects and total ionizing dose effects.
Radiation is referred to the flow of energy emitted from atomic or molecular components when the atomic or molecular components are unstable at higher energy levels. The radiation is represented in the radiation form of an X-ray, a gamma ray, an alpha ray, a beta ray, neutrons, or protons. The radiations are classified into a particle form or an electromagnetic-wave form. The particle form is referred to a particle radiation, and the electromagnetic-wave form is referred to as an electromagnetic-wave radiation. Although the radiations are different from each other, the intensity of the radiation or the influence exerted on an object may be estimated, based on the basic concept of energy flow, depending on the size of an amount of transmitted energy or the size of an amount of absorbed energy.
The radiation may be incident to produce ions, which is called “ionizing radiation”, and other radiations are called “specific ionizing radiations”. In particular, the ionizing radiation causes the damage to a unit MOSFET constituting an electronic part by ionizing atoms of a semiconductor material of the unit MOSFET. Accordingly, the ionizing radiation does not ensure a normal operation of the electronic part and temporarily or permanently damages the electronic part.
FIG. 1 is a view illustrating the configuration of a typical unit MOSFET.
Referring to FIG. 1, the typical unit MOSFET includes a gate to control the operation of the transistor, a drain and a source through which a current flows by the gate, and a body. The thickness of an oxide film of the transistor is 10 nm or more. In this case, when an ionizing radiation is incident to a part having an electromagnetic field, holes may be trapped in the boundary surface between the oxide film and silicon. When the ionizing radiation is incident in the state that a voltage is applied to the gate, the hole trapping is caused at the boundary surface between the drain and the source and thus channel inversion occurs, thereby forming a leakage current path that the current flows. The leakage current path formed by the ionizing radiation causes the abnormal operation of the unit MOSFET. This phenomenon is called “Total Ionizing Dose Effect”.
A PN junction is made between the drain/source and the body of the unit MOSFET. When the ionizing radiation is incident to a part of the PN junction, which is applied with a reverse bias of applying a positive voltage to an N-type part of the PN junction and of applying a negative voltage to a P-type part of the PN junction, electron-hole pairs are produced, electrons and holes move by an electronic field formed by the reverse bias, and thus current pulses flow toward the drain/source and toward the body, respectively. In general, in the state that the reverse bias are applied to the PN junction, the built-in potential becomes greater than potential in an equilibrium state, and thus carriers does not move in an opposite region. Accordingly, a current does not flow. A current pulse, which is produced as the radiation is incident, may affect a circuit including a unit MOSFET, and thus may modulate stored data, which is called a single event effect.
Due to the total ionizing dose effect and the single event effect, the normal operation of the unit MOSFET is not ensured under the radiation environment. In addition, a circuit or a system including such a unit MOSFET may abnormally operate under the radiation environment.
The unit MOSFET employing a dummy gate illustrated in FIG. 3, which serves as a radiation-tolerant unit device, blocks a leakage current path, which is produced due to the total ionizing dose effect, by employing a dummy poly gate layer, a P-active layer, a P+ layer, a dummy metal-1 layer.
In other words, a conventional unit MOSFET using a dummy gate include a dummy poly gate layer for blocking a leakage current path using a phenomenon that hole trapping is not caused when the thickness of an oxide film of a transistor gate is 10 nm or less, and a P-active layer and a P+ layer for blocking a leakage current by preventing channel inversion to be caused by holes trapped by increasing a threshold voltage, in addition to a unit MOSFET including an N-active layer for designating an active region of a transistor such that an isolation field oxide is not produced at a relevant position during process, a poly gate layer for designating a gate region of a transistor by using poly silicon, and an N+ layer for designating an N-type doping position to create a source and a drain in a self-align scheme. In this configuration, the source and the drain of the transistor are surrounded by the dummy poly gate layer, the P-active layer, and the P+ active layer, thereby blocking the leakage current path caused by the radiation.
However, the configuration may minimize only the total ionizing doze effect. When the single event effect occurs, the generated current pulse may affect a circuit.